From the Stargate Wikia (on the SG-1 team):
On Earth, during a mission to expose Ba'al, Mitchell formally re-formed SG-1 with Carter, Jackson, and Teal'c. (SG1: "Ex Deus Machina")
and "Ex Deus Machina":
Lt. Colonel Cameron Mitchell unites Lt. Colonel Samantha Carter, Dr. Daniel Jackson and Teal'c under his command at last
The SG-1 team had split up. Mitchell was given command of the team but needed to find new members. "The big deal" was that he had been trying to reunite the old team, and finally succeeded. The patches didn't replace anything but an empty space, as they were not formal members of any team until then.